This proposal focuses on two primary themes: the ontogenesis of first- order sensory and higher-order integrative areas, and the development of identified serotonergic neurons and their targets. In the lobster brain, these topics are intimately related: identified serotonergic neurons innervate olfactory and integrative neuropil regions early in embryogenesis. This system therefore provides an opportunity to study the developmental assembly of integrative brain regions, and also to investigate the possible serotonergic influence on growth and maturation of those areas. The proposed experiments build on a rich foundation of knowledge about mechanisms of olfaction, hormonal action, and neuromuscular physiology. Aminergic neurons are found in relatively small numbers on both vertebrate and invertebrate nervous systems, have far-reaching projections, and often do not form conventional synaptic contacts. Amine neurons also are thought to have a developmental role in shaping growing neurons and regulating their patterns of connectivity. Because aminergic neurons are involved in a variety of important behavioral processes and clinical disorders, such as sleep, circadian rhythmicity, and mental illness, an understanding of the development of amine neurons and their targets is of critical importance. Using a combination of histological, ultrastructural, immunocytochemical, pharmacological and electrophysiological methods, we propose to (I) characterize the development of the lobster brain, with particular attention to the olfactory and accessory lobes. These regions are serotonin immunoreactive very early in embryogenesis, before their structural organization has been established. (II) determine when and where other transmitter compounds are found in the brain developmentally; (III) analyze the development of the giant serotonergic neurons in the brain that innervate the olfactory and accessory lobes and (IV) test whether serotonin influences the morphogenesis of these regions. Because there are many parallels between the olfactory and accessory lobes of Crustacea and comparable processing areas in the vertebrate brain, and because amines are found in all phyla, these studies will provide knowledge of basic mechanisms that are likely to be relevant to developmental processes in higher organisms as well.